#include <iostream>
using namespace std;

struct X {};
struct Y : X {};

struct Z {
  X& xx;
  Z (X& xxx) : xx (xxx) {}
};

struct ZZ : Z {
  ZZ () : Z (Y()) {}
};

int f (X& x) {} 
int main () {
  Z z (Y());
  //ZZ zz;
}
